AgF-mediated homocoupling reaction of trialkoxy aryl silanes

Article abstract of DOI:10.1080/00397911.2012.736006

In this article, we present the direct homocoupling reaction of trialkoxy aryl silanes mediated by AgF in moderate to excellent yields in very mild conditions. It is an important complement for the synthesis of symmetrical biaryls by using homocoupling of

Full text of DOI:10.1080/00397911.2012.736006

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AgF-Mediated Homocoupling Reaction of
Trialkoxy Aryl Silanes
a
a
Hai-Qing Luo & Wen Dong
a
Department of Chemistry and Chemical Engineering , Gannan
Normal University , Ganzhou , Jiangxi , China
Published online: 08 Jul 2013.
To cite this article: Hai-Qing Luo & Wen Dong (2013) AgF-Mediated Homocoupling Reaction of
Trialkoxy Aryl Silanes, Synthetic Communications: An International Journal for Rapid Communication
of Synthetic Organic Chemistry, 43:20, 2733-2738, DOI: 10.1080/00397911.2012.736006
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Synthetic Communications , 43: 2733–2738, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.736006
AgF-MEDIATED HOMOCOUPLING REACTION OF
TRIALKOXY ARYL SILANES
Hai-Qing Luo and Wen Dong
Department of Chemistry and Chemical Engineering, Gannan Normal
University, Ganzhou, Jiangxi, China
GRAPHICAL ABSTRACT
Abstract In this article, we present the direct homocoupling reaction of trialkoxy aryl
silanes mediated by AgF in moderate to excellent yields in very mild conditions. It is an
important complement for the synthesis of symmetrical biaryls by using homocoupling of
trialkoxy silanes. In this reaction, no other catalyst such as Pd or Cu was necessary.
Supplementary materials are available for this article. Go to the publisher’s online
1
edition of Synthetic Communications for full experimental and spectral details.
Keywords Ag-mediated; biaryls; homocoupling; N,N-dimethylformamide; phenyltri-
methoxysilane
INTRODUCTION
It is well known that biaryls as a central part of organic frameworks have been
[
found in a large number of natural products and widely applied in many useful
1]
[2]
manmade compounds. For more than a century, organic chemists have focused
their attention on developing new and more efficient aryl–aryl bond-forming meth-
[
3]
ods including cross-coupling and homocoupling. Among them, the transition
metal–catalyzed homocoupling reaction is one of the most efficient methods to con-
[
struct symmetrical biaryls using aryl halide or aryl organometallic reagents such as
4]
[
Mg, B, and Sn,
5]
[6]
[3,7]
and transition metal–free oxidative homocoupling reaction
[
8]
has also been studied and established recently. In additional to some methods
[
for aryl–aryl cross-coupling using aryl organosilane reagents and aryl halides, there
are also a number of reports on the homocoupling reaction of organosilanes.
9]
[
10]
Received September 19, 2012.
Address correspondence to Hai-Qing Luo, Department of Chemistry and Chemical Engineering,
Gannan Normal University, Ganzhou, Jiangxi 341000, China. E-mail: luohaiq@sina.com
2
733

2734
H.-Q. LUO AND W. DONG
However, the substrates were limited in the scope of fluorosilane reagents in most
cases. To the best of our knowledge, no report on homocoupling with trialkoxy
silanes has appeared to date.
Compared with similar coupling reactions, using organostannane, Grignard,
and organoboron reagents, Hiyama coupling has many advantages such as low cost,
easy availability, nontoxic by-products, safe handling, and stability in many reaction
conditions. Herein, we present a direct and efficient homocoupling reaction of trialk-
oxy aryl silane reagents mediated by AgF. Such Ag-mediated C-C bond-formation
[10j–k]
reactions are rare.
It is an important complement for the synthesis of symmetri-
cal biaryls through homocoupling reaction of trialkoxy aryl silanes. In this reaction,
no other catalyst such as Pd or Cu was necessary.
RESULTS AND DISCUSSION
In our initial screening of various fluorides, it was gratifying to find that
AgF could afford the desired homocoupling product in excellent yield using
a
Table 1. Optimizing the reaction conditions for the direct homocoupling of phenyltrimethoxysilane
ꢀ
b
Yield (%)
Entry
Additives
Solvent
Temp. ( C)
1
2
3
4
5
KF
Dioxane
Dioxane
Dioxane
Dioxane
DMF
80
80
80
80
80
80
80
80
80
60
80
rt
0
0
c
TBAF
CsF
CuF
0
2
0
PdCl (10% mol)
PdCl (TBAF)
CuCl (1.0 equiv)
2
0
d
6
2
DMF
DMF
6
7
0
e
8
CuI or CuBr (TBAF)
DMF
DCE
THF
0
9
AgF
AgF
AgF
AgF
AgF
0
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
0
1,4-Dioxane
DMF
DMF
72
95
96
48
92
95
61
80
80
80
80
rt
f
AgF
DMF
DMF
g
AgF
h
AgF
AgF
DMF
DMF
i
a
b
c
Reactions were carried out with 1.05 equiv of fluoride, 2.0 mL solvent, 8 h.
Isolated yield based on phenyltrimethoxysilane.
TBAF ¼ tetra-n-butylammonium fluoride.
d
2
10 mol% PdCl and 2.0 equiv TBAF.
e
1
0 mol% CuCl or CuI and 1.1 equiv TBAF.
f
0
.5 equiv AgF was used.
g
h
i
1.0 equiv AgF was used.
1.4 equiv AgF was used.
Using phenyltriethoxysilane as substrate.

AgF-MEDIATED HOMOCOUPLING
2735
ꢀ
N,N-dimethylformamide (DMF) as solvent at 80 C (Table 1, entry 13). It can
be seen from Table 1 that fluorides such as KF, CsF, CuF , and TBAF (tetra-n-
butyammonium fluoride) were inactive in this system (Table 1, entries 1–4). Only
2
6
entry 6), and no desired product was obtained while only PdCl (10% mol) was used
% yield was obtained by adding catalytic PdCl (10% mol) and fluoride (Table 1,
2
2
as catalyst (Table 1, entry 5).
Even when using 1.0 equivalent of CuCl, or TBAF combined with catalytic
CuBr or CuI (Table 1, entries 7, 8), no desired product was obtained. The reactions
were performed in different solvents such as 1,2-dichloroethane (DCE), THF, 1,4-
dioxane, and DMF (Table 1, entries 9–12). It found that DMF is the most suitable
a
Table 2. AgF-mediated homocoupling reaction of various trialkoxy aryl silanes
b
Entry
1
Time (h)
2
Yield
1
2
3
4
4
6
2a
2a
2b
2b
96
82
10
16
87
69
5
6
12
20
2c
2d
63
48
7
8
9
12
24
12
8
2e
2e
97
88
89
96
82
2f
1
0
1
2 g
2 h
1
8
a
Reactions were carried out with 1.05 equiv of silver fluoride, 2.0 mL DMF.

2736
H.-Q. LUO AND W. DONG
Scheme 1. AgF-mediated homocoupling reaction of trialkoxy aromatic silanes.
choice for this homocoupling reaction although 1,4-dioxane is also favored for this
reaction with the desired product obtained in good yield (Table 1, entry 11). The
loading of AgF could be further examined, and 1.05 equivalents of AgF was found
necessary to complete the consumption of substrate (Table 1, entries 13–16). In
addition, using phenyltrimethoxysilane as substrate, the reaction proceeded smoothly
at room temperature in excellent yield (Table 1, entry 12). However, when phenyl-
triethoxysilane was tested as substrate at room temperature, the yield decreased sig-
nificantly (Table 1, entry 17).
With these optimized conditions in hand, we further explored the scope of this
ꢀ
homocoupling reaction with various trialkoxy aryl silanes in DMF at 80 C in the
presence of 1.05 equivalents of AgF. The results are summarized in Table 2. To
our delight, the biaryl products were obtained in moderate to excellent yields by
the homocoupling of different trialkoxy aryl silanes having electron-donating or
electron-withdrawing groups on the phenyl rings. Triethoxyphenylsilane was found
to be less reactive than trimethoxyphenylsilane. Notably, the yields decreased
dramatically by using triethoxyphenylsilane bearing the o-methyl group (Table 2,
entry 6), which could be due to the steric effect of the methyl substituent adjacent
to the triethoxylsilyl group on the phenyl ring.
No homocoupling product was detected when benzylic silane 1m was treated
with AgF under the same condition (Scheme 1).
In correlation with Kochi’s proposal for the coupling of Grignard reagents and
[7]
alkyl halides, we propose the possible mechanism to account for the aryl-aryl
homocoupling products (Scheme 2). We envisaged that, in this coupling reaction
process, AgF plays a role not only as a simple fluoride to activate the trialkoxy aryl
silane but also as an oxidant to complete this aryl–aryl homocoupling reaction. The
fluorinated intermediate 3 was rationally generated with the trialkoxy aryl
ꢁ
þ
I
silane activated by F , which underwent oxidation by Ag to produce aryl-Ag
Scheme 2. Proposed mechanism of homocoupling of trialkoxy aryl silane mediated by AgF.

AgF-MEDIATED HOMOCOUPLING
2737
(intermediate 5). Finally, the biaryl product of homocoupling reaction was generated
along with the formation of the silver metal from the intermediate 5.
CONCLUSION
In conclusion, we have developed a direct entry into symmetrical biaryls by
homocoupling of aryl organosilane reagents mediated by AgF. It is important comp-
lement to synthesis of symmetrical biaryls using homocoupling of trialkoxy silanes.
In this reaction, no other catalyst such as Pd or Cu was necessary. Futher extension
of this work using catalytic silver salt is currently in progress.
EXPERIMENTAL
To an oven-dried, 10-mL, round-bottom flask equipped with a magnetic stirring
bar were added AgF (67.2 mg, 0.525 mmol, 1.05 equiv) and phenyltriethoxysilane
(120 mg, 0.5 mmol, 1.0 equiv) in 2 mL anhydrous DMF. Then the mixture was stirred
for 6 h at 80 C before the mixture was quenched with H O (5 mL). The aqueous layer
ꢀ
2
was extracted with diethyl ether (10 mL ꢂ 3), and the combined organic layers were
washed with brine, dried over anhydrous MgSO , filtered, and concentrated in vacuo.
4
The residue was purified by flash column chromatography to afford the desired pru-
1
duct 2a (31.6 mg) as white solid. H NMR: d (400 MHz, CDCl ) 7.59 (d, J ¼ 8.4 Hz,
3
1
3
4
H), 7.44 (t, J ¼ 8.0 Hz, 4H), 7.39 (t, J ¼ 7.6 Hz, 2H); C NMR (100 MHz, CDCl ):
3
1
41.2, 129.9, 128.7, 127.2.
Please see the Supporting Information, available online, for complete experi-
mental and spectral details.
ACKNOWLEDGMENTS
This project was supported by the Natural Science Foundation of Jiangxi,
China (No. 2008GZH0014). The authors aslo thank Gannan Normal University
for financial support.
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¨
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1